Combine mayonnaise, garlic, ground pepper, and cayenne pepper in medium bowl and whisk to blend. Mix in cheeses; season with salt. (Can be made 2 days ahead. Cover and refrigerate.)
Preheat oven to 400°F. Brush 1 side of each bread slice with oil. Place oiled side down on large rimmed baking sheet. Bake bread slices until almost golden, about 5 minutes. Spread 1 generous tablespoon cheese mixture on each bread slice on sheet. Bake until cheese topping starts to bubble and toasts are golden at edges, about 10 minutes.
Transfer toasts to large platter. Sprinkle with chopped green onions and serve warm.